Chemical tools play essential roles in dissecting the molecular mechanisms underlying biological processes in living systems.In the history of plant biology, N-1-naphthylphthalamic acid (NPA) has long been used as an auxin export inhibitor, and multiple in vivo targets have been proposed, such as ABC transporters,TWISTED DWARF1, and BIG/-rlR3 (reviewed in Teale and Palme, 2018).These proteins exhibit pleiotropic functions, which were proposed to indirectly mediate the NPA effect through the activity or trafficking of PIN-FORMED (PIN) auxin transporters.However, the debate on the pure target of N PA often makes a clear interpretation of such experimental data elusive.